Recently, materials that use recyclable natural fibers have received attention due to increasing environmental awareness and for the purpose of substituting petroleum resources. Among natural fibers, a cellulose fiber having a fiber diameter of 10 to 50 μm, particularly, a tree-derived cellulose fiber (pulp), has been widely used mainly as a paper product. In addition, cellulose fibers that are fine fibers having a fiber diameter of 1 μm or less are also known, and a sheet containing these fine fibers has advantages such as a high mechanical strength. Therefore, applications of these to various purposes have been investigated. For example, a use of the fine fiber where the fine fiber is made into a non-woven fabric and utilized as a high-strength sheet has been known. In addition, when the fine fiber is composited with a polymer, heat resistant dimensional stability is significantly enhanced due to more uniform, finer dispersion of the fine fiber in the polymer. Such composite materials can be used in various structural members, and great expectation surrounds their applications as organic ELs or flexible transparent substrates for liquid crystal displays.
Patent Document 1 and Patent Document 2 disclose methods of producing fine fibers that micronize fibers by utilizing a function of cellulase enzyme that selectively cuts the amorphous region of a cellulose fiber and a function of xylanase or hemicellulase that selectively cuts a xyloglucan or hemicellulose component that serves as an adhesive between microfibrils.
Patent Document 3 and Patent Document 4 attempted micronizing the fiber by using an endo-glucanase type cellulase enzyme.
In addition, fine fibrous cellulose having a fiber diameter in the order of nanometers is known as a cellulose fiber (Patent Documents 5, 6, and 7). For example, Patent Document 5 discloses a fine fibrous cellulose having a degree of polymerization of 500 or greater obtained by fibrillating beaten pulp. Patent Document 6 discloses a fine fibrous cellulose having a degree of polymerization of 600 or greater obtained by fibrillating a cellulose raw material in an ionic liquid. Patent Document 7 discloses a fine fibrous cellulose obtained by treating a cellulose raw material with N-oxyl and a cooxidation agent such as sodium hypochlorite to fibrillate the cellulose raw material. The treatment using N-oxyl and a cooxidation agent of Patent Document 7 oxidizes a hydroxy group of cellulose to form carboxy group.
Recently, applications of fine fibrous cellulose to various purposes have been investigated. For example, it has been investigated in order to obtain a fiber reinforced resin composite by dehydrating after mixing fine fibrous cellulose with an emulsion resin.